The Winter King by Bernard Cornwell

Essentially this is a modern political thriller, told in flat American diction. Narrated by Derfel, an ordinary, likable man who rises through the ranks to become Arthur's friend and advisor in peace and war, the story doesn't follow the traditional patterns. Mordred is Uther's infant grandson, the legitimate king; Arthur is one of Mordred's guardians, sworn to hold the kingdom against the Saxon warlords until Mordred comes of age. Warfare is incessant. Arthur's dream of peace and unity seems unattainable. Derfel's own story--his strange origin, his love for Nimue, his worries and his triumphs--parallels Arthur's as he fights for and beside him.